Singleton objects are not destroyed on exit

Singleton objects are not destroyed on exit, such as QQuickControlSettings object.
This issue was introduced by 6fc1e0fc36f0732e79375a0e105816eb17893962, with this patch QQmlType::SingletonInstanceInfo::destroy() will run the following code:

    QObject *o = qobjectApis.take(e);
    if (o) {
        QQmlData *ddata = QQmlData::get(o, false);
        if (ddata && ddata->indestructible)
            return;
        delete o;
    }

On exit, ddata->indestructible is true and we don't destroy the Singleton object and we will never have a chance to do it.